Coincidental Plagiarism

This shot has a real embarrassing story behind it. I just recently got a gig to work on a logo design for an e-wallet (a company name starting with Z).I didn't have any idea that this design was already existing (the shot which I'm rebounding), and I submitted these two color option to the client and client liked it but later he found the design of Denis Zanker and he accused me of plagiarism. And in short, I lost that gig and left with total humiliation from the client. Even though I was totally unaware of the existence of the existing Zanker's logo, I think I owe an apology to the original creator for copying his work ...